1. Field of the Invention
This invention relates generally geologic surveys and more particularly to an apparatus and method for acquiring and processing seismic data.
2. Description of the Related Art
Conventional geophone, accelerometer and hydrophone systems used in seismic prospecting typically have several sensors that produce signals indicative of acoustic energy usually in the form of a seismic wave. The seismic wave is usually produced by an energy source such as a vibrator truck, explosives or by an air gun in the case of a hydrophone system. These seismic signals are then conducted to acquisition/conversion circuitry. The analog signals from one or more remote seismic sensors (hydrophones, geophones, or other seismic sensors) are sampled and converted to a series of digital values by the acquisition/conversion circuitry. The acquisition/conversion circuitry is typically configurable to, for example, adjust the sampling rate, alter any digital filtering or other digital signal processing parameters, or perform diagnostics.
One or more of these acquisition/conversion circuits are connected to a data collection unit. Each data collection unit collects the series of digital values for all the seismic sensors connected to all the acquisition/conversion units connected to it. The data collection unit passes that data to a system controller, which may include a seismic recording device.
Acoustic seismic waves include a variety of energy signatures. They include P & Shear signals that are characterized by differing arrival times (differing velocities) at the detectors. The signals contain different frequencies for any given geologic event. Hence, they do not correlate between a time limited vertical and horizontal time window.
Other seismic signals are less desirable signals known as noise. One form of noise is known as ground roll. Ground roll noise, or simply ground roll arrives at vertical and horizontal components at the same time with the same frequencies. These signals are polarized at 90 degrees to each other. Therefore, the noise does correlate between time limited vertical and horizontal records.
Ground roll is generated when the acoustic source is activated. The acoustic energy reflects from shallow impedances as well as impedances at a target depth. The shallow reflections have large amplitude as compared to reflections from the target depth, and sometimes arrive at surface detectors at the same time. Noise issues adversely affect seismic data acquisition to the extent that many conventional systems are designed around noise characteristics rather than seismic signal needs. One example is the generally accepted practice of deploying geophones in arrays in an effort to cancel the effects of ground roll based on the fact that these noise signals have different frequencies than signal events (band limited at least), are large amplitude and are dispersive. Using arrays alone to reduce ground roll is only partially successful at ground roll elimination. Additionally, the slow velocity of ground roll requires strict spatial sampling requirements if it is to be eliminated using traditional filtering approaches such as frequency-wave number (FK) filtering.
Polarization filters are the broad category of filters that rely on detection and separation of ground roll due to its elliptical particle motion. Several polarization filters have been used with only limited success because of the dispersing nature of ground roll as well as the invalidation of many assumptions due to local geologic conditions.
Adaptive systems are generally characterized by the ability to adjust performance in a time-varying fashion using a dynamic noise estimate input. Known adaptive filtering techniques do not include the ability to use both the vertical and horizontal components from a single multi-axis sensor. Moreover, the typical filtering methods do not differentiate ground roll from other forms of energy and may filter desirable signals as well as ground roll. Finally, the typical system does not provide the ability to reduce ground roll station-by-station, because of the general reliance on spatial sampling and receiver arrays.